This invention relates to a frequency domain optical processor for producing imagery in a sidelooking syntheticaperture radar system, and more particularly to the use of a spatial matched filter in the production of the imagery through interferometry.
Air-borne radar systems have been developed for imaging of the earth's surface using a small, sidelooking antenna that scans an area alongside the aircraft's track coordinate, herein referred to as the azimuth coordinate. The antenna of the system radiates and receives a beam of RF pulses. The beam pattern is relatively wide along the aximuth coordinate which scans the terrain by virtue of the forward motion of the aircraft. In the cross-track coordinate, herein referred to as the slant-range coordinate, the radiated pulses scan the illuminated areas of the terrain in a direction orthogonal to the azimuth coordinate by virtue of the differential time delay. Since the area illuminated by the beam is disposed to one side of the azimuth coordinate, the slant-range coordinate has a simple relation to range measure along the earth's surface.
In order to provide radar imagery having angular resolution capabilities comparable with photo-reconnaissance systems, the azimuth dimension of the radar beam must be very large, particularly as the distance from the antenna to the target increases. It is in fact known that to have an acceptable azimuth resolution at long ranges, the length of the antenna aperture in the radar system can reach impractical lengths which, of course, an aircraft could not carry.
A known synthetic-aperture technique provides a synthesized antenna of suitable length which will provide the required resolving power. Thus, although the sidelooking antenna, which in itself is relatively small, scans the terrain continuously, it may be considered as if it scans the terrain at discrete positions which are determined by the forward speed of the aircraft. Accordingly, each of the discrete positions operates as a single element in a large antenna array. At each of these positions, the antenna radiates and receives a signal which is stored, the stored signals are then processed in a manner analogous to the physically large array. The resultant output signal of the small sidelooking antenna is characterized by an improved signal-to-noise ratio as well as an improved target resolving capability which are characteristics to be found in an output signal obtainable from the large antenna structure.
Summation of the discrete signals developed in the synthetic-aperture radar system is performed in one instance by way of a photographic technique which provides high-density storage together with fine-resolution imaging. A known method employs the radar receiver output, which comprises a series of reflected range pulses, to intensity modulate the beam of a cathode-ray tube. The beam is swept vertically and corresponds in scale to the slant-range coordinate, thus providing a single line trace. A film strip is transported past the trace to record the beam intensity variations. Thus, data information is recorded in a two dimensional format, the dimension across the film representing range, and the longitudinal direction of the film corresponding to the azimuth. In this respect, it will be noted that the time variable has been converted to a space variable that is defined in terms of film length.
The use of photographic film as a store for the radar data provides a convenient solution to the problems associated with storage and manipulation of high resolution signal information. However, these advantages are obtained in exchange for problems experienced when processing the recorded raw data to obtain a reproduced photographic image like that of a conventional aerial photograph. For example, since the target area is illuminated by the radar beam at an angle of incidence less than 90.degree. and has a slant-range coordinate, the recorded radar signal image is tilted with respect to the plane of the recording film so that returning signals having a short slant-range coordinate produce signal recordings having shorter focal lengths. Conversely, an RF pulse returning to the antenna along a longer slant-range coordinate produces a signal record image having a longer focal length. It will be understood that these focal properties apply only in the azimuth direction and that in the range direction resolution is obtained by pulse modulation of the radar signals. The image in range is thus in a plane parallel to the film record whereas the azimuth image is in a plane tilted with respect to the plane of the recording film.
Optical processor systems in the prior art are known which bring the foregoing two planes into coincidence. In one known system, a complex lens structure was used which in appearance comprised a vertical section of a cone and was referred to as an "axicon" lens. Processors employing the axicon lens were subsequently supplanted by a tilted-plane processor which provided more versatility and considerably improved imagery. A third method involved a matched filtering operation in the spatial frequency domain. This latter method has all of the advantages of the tilted plane processor together with a further advantage, that readily available and inexpensive optical elements may be employed.
Various attempts were made to develop an appropriate spatial filter, one of which involved putting two cylindrical lenses in tandem with one of the lenses being appropriately tilted. Another solution was a computer-generated matched filter. However, a limitation of the computer-generated matched filter technique is that inadequate resolution is obtained through inherent limitations in the method which limits the accuracy and quality of the filter. In order to alleviate this deficiency, an improvement in resolution was sought by substituting computer generation of the filter with an interferometric technique.